This invention relates to a balanced filter circuit comprising filter inputs for supplying an input signal and filter outputs for deriving an output signal, the filter circuit comprising
a balanced amplifier having an inverting input and a non-inverting input which are coupled to the filter inputs, and an inverting output and a non-inverting output which are coupled to the filter outputs; and PA1 at least one pair of filter resistors comprising a first and a second filter resistor.
Such a filter circuit is known from U.S. Pat. No. 4,509,019. The filter circuits shown therein are composed of a number of balanced amplifiers with filter resistors and filter capacitors connected pairwise thereto. In order to be able to tune the filter, in the known filter circuit all of the filter resistors are in the form of MOS transistors, whose resistance is adjusted by a variable voltage at the control electrodes of the MOS transistors. This method allows only a comparatively small variation of the filter resistances so that only a limited tuning range of the filter is obtained. An alternative method is known from European Patent Application 0 270 156, which corresponds to U.S. Pat. No. 4,780,690. In this case, the filter resistors are in the form of a transconductance circuit, whose transconductance is adjusted by an adjustable current source. The transconductance circuit behaves like a pair of filter resistors whose value is inversely proportional to the transconductance. The transconductance and hence the resistance value of such a transconductance circuit can be controlled over a comparatively wide range. Since for each pair of resistors a transconductance circuit with an adjustable current source is required, with more complex filter circuits the number of electronic components for the transconductance circuits and hence the current consumption related to these circuits increase.